Allow cloning volumes with no capacity specified

In virStorageVolCreateXML, add VIR_VOL_XML_PARSE_NO_CAPACITY
to the call parsing the XML of the new volume to make the capacity
optional.

If the capacity is omitted, use the capacity of the old volume.
We already do that for values that are less than the original
volume capacity.

tests/storagevolxml2xmlout/vol-qcow2-nocapacity.xml

@@ -0,0 +1,21 @@
+<volume type='file'>
+  <name>OtherDemo.img</name>
+  <key>/var/lib/libvirt/images/OtherDemo.img</key>
+  <source>
+  </source>
+  <capacity unit='bytes'>0</capacity>
+  <allocation unit='bytes'>0</allocation>
+  <target>
+    <path>/var/lib/libvirt/images/OtherDemo.img</path>
+    <format type='qcow2'/>
+    <permissions>
+      <mode>0644</mode>
+      <owner>0</owner>
+      <group>0</group>
+      <label>unconfined_u:object_r:virt_image_t:s0</label>
+    </permissions>
+    <encryption format='qcow'>
+      <secret type='passphrase' uuid='e78d4b51-a2af-485f-b0f5-afca709a80f4'/>
+    </encryption>
+  </target>
+</volume>